std::pmr::polymorphic_allocator<T>::construct

Since C++17

// 1)
template < class U, class... Args >
void construct( U* p, Args&&... args );

Since C++17, Until C++20

// 2)
template< class T1, class T2, class... Args1, class... Args2 >
void construct( std::pair<T1, T2>* p,
                std::piecewise_construct_t,
                std::tuple<Args1...> x,
                std::tuple<Args2...> y );
// 3)
template< class T1, class T2 >
void construct( std::pair<T1, T2>* p );
// 4)
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, U&& x, V&& y );
// 5)
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, const std::pair<U, V>& xy );
// 6)
template< class T1, class T2, class U, class V >
void construct( std::pair<T1, T2>* p, std::pair<U, V>&& xy );
// 7)
template< class T1, class T2, class NonPair >
void construct( std::pair<T1, T2>* p, NonPair&& non_pair );

Constructs an object in allocated, but not initialized storage pointed to by p the provided constructor arguments. If the object is of type that itself uses allocators, or if it is std::pair, passes *this down to the constructed object.

1. Creates an object of the given type U by means of uses-allocator construction at the uninitialized memory location indicated by p, using *this as the allocator.
   This overload participates in overload resolution only if U is not a specialization of std::pair. (until C++20)

2. First, if either T1 or T2 is allocator-aware, modifies the tuples x and y to include this->resource(), resulting in the two new tuples xprime and yprime, according to the following three rules:

   • 2a) if T1 is not allocator-aware (std::uses_allocator<T1, polymorphic_allocator>::value==false) and std::is_constructible<T1, Args1...>::value==true, then xprime is x, unmodified.

   • 2b) if T1 is allocator-aware (std::uses_allocator<T1, polymorphic_allocator>::value==true), and its constructor takes an allocator tag (std::is_constructible<T1, std::allocator_arg_t, polymorphic_allocator, Args1...>::value==true), then xprime is std::tuple_cat(std::make_tuple(std::allocator_arg, *this), std::move(x))

   • 2c) if T1 is allocator-aware (std::uses_allocator<T1, polymorphic_allocator>::value==true), and its constructor takes the allocator as the last argument (std::is_constructible<T1, Args1..., polymorphic_allocator>::value==true), then xprime is std::tuple_cat(std::move(x), std::make_tuple(*this)).

   • 2d) Otherwise, the program is ill-formed.

   Same rules apply to T2 and the replacement of y with yprime. Once xprime and yprime are constructed, constructs the pair p in allocated storage as if by ::new((void *) p) pair<T1, T2>(std::piecewise_construct, std::move(xprime), std::move(yprime));

3. Equivalent to construct(p, std::piecewise_construct, std::tuple<>(), std::tuple<>()), that is, passes the memory resource on to the pair's member types if they accept them.

4. Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(x)),
                                       std::forward_as_tuple(std::forward<V>(y)))

5. Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(xy.first),
                                       std::forward_as_tuple(xy.second))

6. Equivalent to

construct(p, std::piecewise_construct, std::forward_as_tuple(std::forward<U>(xy.first)),
                                       std::forward_as_tuple(std::forward<V>(xy.second)))

7. This overload participates in overload resolution only if given the exposition-only function template

template< class A, class B >
void /*deduce-as-pair*/( const std::pair<A, B>& );

, /*deduce-as-pair*/(non_pair) is ill-formed when considered as an unevaluated operand. Equivalent to

construct<T1, T2, T1, T2>(p, std::forward<NonPair>(non_pair));

 (until C++20)

Parameters

p - pointer to allocated, but not initialized storage
args... - the constructor arguments to pass to the constructor of T
x - the constructor arguments to pass to the constructor of T1
y - the constructor arguments to pass to the constructor of T2
xy - the pair whose two members are the constructor arguments for T1 and T2
non_pair - non-pair argument to convert to pair for further construction

Return value

(none)

Notes

This function is called (through std::allocator_traits) by any allocator-aware object, such as std::pmr::vector (or another std::vector that was given a std::pmr::polymorphic_allocator as the allocator to use).

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2969	C++17	uses-allocator construction passed resource()	passes *this
LWG 2975	C++17	first overload is mistakenly used for pair construction in some cases	constrained to not accept pairs
LWG 3525	C++17	no overload could handle non-pair types convertible to pair	reconstructing overload added